This invention relates to massagers in general and, in particular, to those of self-excited vibration variety. More particularly, the invention pertains to a hand-held massager featuring a closed-loop electropneumatic vibration control system for positive feedback of the mechanical impedance of the object of massaging (usually, some body part) to a vibrator. This vibrator is therefore self-excited to vibrate at a resonance frequency of the object and hence to massage the same at the frequency optimal to that particular object.
Vibrating massagers, having a mechanically vibrating part or parts to be held against desired body parts, are well-known. In designing a massager, one can ascertain the resonance frequency of the mechanical impedance of a particular body part, and massage the object at that frequency. See Japanese Unexamined Patent Publication No. 58-138455, which discusses attaching both acceleration and pressure sensors to the part that is to be held against the body. The mechanical impedance of the desired body part is obtained by sweeping the frequency of vibration, and the vibrator is driven at the resonance frequency of the mechanical impedance that has been computed and memorized after the measurement.
In Japanese Unexamined Patent Publication No. 2001-252325, a sensor rather than a vibrator is held against the desired object for impedance measurement. The mechanical impedance of the object is positively fed back from the sensor to the vibrator thereby causing the latter to vibrate at the desired resonance frequency.
Japanese Unexamined Patent Publication No. 6-327639 discusses how to measure the elasticity or like parameter of a human skin surface using a hollow vibrator.
The first cited patent application, teaching to sweep through an expected range of vibration frequencies for determining the resonance frequency of the desired object seems undesirable because the resonance frequency is susceptible to substantive variation depending upon how the vibrator contacts the body surface. The one being massaged is thoroughly restrained during measurement, and not allowed to move. In event he or she does move, moreover, the resonance frequency must be recalibrated.
An additional objection arises from the necessity of mounting the sensors in close proximity of that part of the massager which makes direct contact with the human body. Being most exposed and most often held against the human body, this part is most susceptible to impairment or damage, both physically and chemically. The sensors positioned close to such a vulnerable part are easy to suffer impairment or total destruction, demanding much time and cost for upkeep.
In Japanese Unexamined Patent Publication No. 2001-252325, in which the resonance frequency is automatically renewed by positive feedback, the sensor is not united with the massager. However, one must fasten the sensor to the desired body part either with heavy-duty double-sided adhesive tape or with a belt. Use of such tape or belt is not only troublesome and time-consuming but may cause skin irritation. Certain limitations are imposed, moreover, upon the posture of the user. Furthermore, the sensor carries live current, and therefore poses an additional risk to the user.